1. Field of the Invention
The present invention relates to a method for manufacturing a boundary acoustic wave device, for use in a resonator or a band-pass filter, for example. More specifically, the present invention relates to a method for manufacturing a boundary acoustic wave device having a structure in which a film unevenness of a dielectric substance is improved.
2. Description of the Related Art
Surface acoustic wave devices are widely used for band-pass filters of cellular phones or the like. On the other hand, instead of such surface acoustic wave devices, boundary acoustic wave devices have been studied for simplifying and downsizing of a package structure.
WO2004/070946 discloses one example of a method for manufacturing a boundary acoustic wave device. In the manufacturing method described in WO2004/070946, first, an electrode including an IDT is formed on a piezoelectric substrate composed of LiNbO3 or the like by photolithography. Subsequently, a dielectric film composed of, for example, SiO2, is formed by a deposition method such as sputtering, vapor deposition, or CVD so as to cover the IDT. In the thus-obtained boundary acoustic wave device, a boundary acoustic wave is excited by the IDT at a boundary between the piezoelectric substrate and the dielectric film and propagates through the boundary.
The IDT includes a plurality of electrode fingers and there are spaces between the electrode fingers where the electrode is not present. Therefore, there is a difference in height between the region where the electrode is present and the region such as the spaces where the electrode is not present. When the dielectric film is formed by the deposition method, the dielectric film tends to grow in a tilted manner or an uneven dielectric film tends to be formed by the difference in height between the region where the electrode is present and the region where the electrode is not present. As a result, the characteristics of the obtained boundary acoustic wave device may deteriorate.
To solve the problems described above, WO2004/070946 describes that decreasing of the thickness of the electrode such as the IDT is effective. Specifically, it is described that, assuming that a wavelength of a boundary acoustic wave is denoted by λ, the thickness H of the IDT is preferably 0.1λ or less.
However, even if the thickness of the IDT is about 0.1λ as described in WO2004/070946, elastic discontinuous portions tend to be formed in the dielectric film at centers of the spaces between the electrode fingers. This will be described with reference to FIGS. 10 and 11.
FIGS. 10 and 11 are schematic front sectional views showing the presence of elastic discontinuous portions in a SiO2 film when a boundary acoustic wave device is manufactured in accordance with the following specifications.
a) Specifications of the Boundary Acoustic Wave Device Shown in FIG. 10
An IDT with λ=3.6 μm, thickness=0.103λ, and duty=0.6 was formed on a LiNbO3 substrate. A SiO2 film having a thickness of 6 μm was further formed by RF magnetron sputtering.
As evident from FIG. 10, an IDT 102 is disposed on a LiNbO3 substrate 101. The IDT 102 includes electrode fingers 102a and 102b. A SiO2 film 103 is formed so as to cover the IDT 102. An elastic discontinuous portion 103a is created in the SiO2 film 103. The elastic discontinuous portion 103a is positioned above the center of a space between the electrode fingers 102a and 102b. The elastic discontinuous portion 103a is present at a position of 0.18λ to 0.348λ above the upper surface of the LiNbO3 substrate. The elastic discontinuous portion 103a is believed to be formed by the collision between the SiO2 film deposited on an electrode finger 102a side and the SiO2 film deposited on an electrode finger 102b side at the center of the space, when the SiO2 film is formed by deposition.
The presence of the elastic discontinuous portion 103a may deteriorate the electrical characteristics of the boundary acoustic wave device when a boundary acoustic wave is excited.
Specifically, the presence of the elastic discontinuous portion 103a may provide a boundary acoustic wave with an uneven acoustic velocity and deteriorate the electromechanical conversion efficiency provided by the IDT 102. Consequently, an electromechanical coefficient may decrease or a propagation loss or a frequency variation may increase.
b) Specifications of the Boundary Acoustic Wave Device Shown in FIG. 11
An IDT with λ=1.6 μm, thickness=0.113λ, and duty=0.5 was formed on a LiNbO3 substrate 201. A SiO2 film 203 having a thickness of 6 μm was further formed.
In this case, elastic discontinuous portions 203a to 203c are created in the SiO2 film above the spaces between electrode fingers 202a and 202b, between electrode fingers 202b and 202c, and between an electrode finger 202c and an outer electrode finger (not shown), respectively.
The elastic discontinuous portions 203a to 203c are present at the position between 0.15λ and 0.46λ to 0.5λ above the upper surface of the LiNbO3 substrate 201.
As shown in FIGS. 10 and 11, even if the thickness of the IDT is set to be about 0.103λ or 0.113λ, the large elastic discontinuous portions 103a and 203a to 203c are created in the SiO2 film, which may deteriorate the electrical characteristics of the boundary acoustic wave device.
As described in WO2004/070946, if the thickness of the IDT is set to be 0.1λ or less, that is, the thickness is further decreased, the effect due to the thickness of the IDT can be suppressed. However, if the thickness of the IDT is excessively decreased, an electrical resistance is increased or a sufficiently large electromechanical coefficient cannot be achieved.